Spectroscopy is the science of measuring radiation from a source. The measured input is comprised of a range of frequencies or wavelengths which are recorded and represented as a spectrum. Spectroscopy can involve different ranges of radiation including microwaves, visible and invisible light, X-rays, or gamma rays, as well as other types of signals. In nuclear spectroscopy, the X-rays and gamma rays are frequently used to identify the presence of specific radioactive materials; this information is useful in determining both the type (natural, benign, threatening) and quantity (safe level, level of concern, health threat) of the material present.
Neutron radiation is another form of radiation, normally produced by the breakdown of atomic nuclei in fission, or by various neutron generation methods. When identifying radioactive materials, the presence of neutrons provides a critical factor in detecting and discriminating certain isotopes, particularly special nuclear materials (SNM). Neutron energies may be measured spectroscopically, but while there are many applications for this, isotope identification normally relies on simple neutron detection and counting, rather than spectroscopy.
Neutron detection is used in national security (e.g. protection against nuclear terrorism), scientific research (e.g. neutron scattering for materials research), health physics (e.g. monitoring and control of personnel exposure at nuclear power plants), and other applications. Neutron detector requirements vary according to the application and specific intended use and can range from simple counting to detecting the presence of a neutron source and providing information about its identity and location. In general, most neutron detectors do not perform in an optimal way for their intended use and the performance of most neutron detectors is well below that of theoretical limits. An example of this is the type of neutron detector used in radiation portal monitors.
Ideally, one would want to detect 100% of the neutrons emitted by a neutron source present in the object being scanned (e.g. a vehicle or cargo container) as this would maximize the likelihood of the portal monitor determining that the source was present. For neutron detection, most portal monitors use a neutron detector that consists of one or more 3He proportional counters embedded in a blanket of neutron moderator material (e.g. high-density polyethylene, or HDPE). For most current systems, a fast neutron (e.g. energy between 100 keV and 20 MeV) entering the surface of the device orthogonally has a probability of being captured and detected in the 3He counter of between 15 and 20%. Not only would one want to know whether or not a source is present, but ideally one would also like to know what type of source it is (e.g. potentially threatening or not), how big it is, it's location, etc.
There are many devices known in the art which are capable of gamma-ray spectroscopy, but which do not have neutron detection capabilities. In addition, other systems have been installed with neutron detection capabilities, but because gas proportional detectors may gradually degrade through the loss of gas, particularly those utilizing 3He because of its high diffusion rate, spectrometers have often degraded or are inoperable due to failed neutron detectors which are in need of repair. The spectrometer apparatus apart from the neutron detector portion may still be operational and due to the considerable expense and value of these devices, it is not something to be casually discarded and thrown away due to the breakdown of the neutron detector. Thus, there is a clear need for a method and apparatus that can add or restore neutron detection abilities using existing spectrometer equipment.
It is well known that 3He proportional counters have been the favored technology for most large area systems (including area and portal monitors) in most situations where the presence of neutrons is the critical measurement, and where the energy and direction of the neutron source is less critical. This form of detector can be made very sensitive (by increasing pressure), fairly large (by building large tubes), and is very immune to false positives from high gamma exposure (“gamma crosstalk”). Unfortunately, because of the high diffusion rate of helium gas, especially under high differential pressure, these detectors tend to fail over extended periods of time and require refurbishment or replacement.
In recent years, 3He stocks have been rapidly depleted due to the combined effects of the upsurge in demand after the terrorist attacks of Sep. 11, 2001, the Helium Privatization Act of 1996, and the diminishing number of tritium-bearing warheads being disassembled (tritium may be used to produce 3He). As a result, it is becoming more and more difficult to repair and service existing systems using 3He neutron detectors. In this context, the disclosed invention provides an attachment that can measure neutrons without relying on 3He, using only detectors that are already part of installed equipment plus a small amount of electronics. This provides the user with a unique, inexpensive solution to a common problem—how to repair an existing monitoring system without expensive or unavailable 3He.
A great deal of research and development has been expended over the years in the pursuit of improved neutron detectors and many different detection methods have been investigated. Although current devices are far more sophisticated and have much better performance than their predecessors of several decades ago, few solutions exist which are fully commercialized. Even those that have been commercialized are technically complex by comparison to 3He, and most suffer from issues regarding gamma crosstalk as described above. The most widely deployed at this time are Li-based scintillators (which require complex discrimination to eliminate gamma crosstalk) and BF3 tubes, which are considered so hazardous that most customers reject them.
Traditionally, neutron detectors have been arranged in a generally cylindrical geometry with a central detecting element such as a gas proportional tube or a bundle of fiber optic scintillators, surrounded by a generally annular body of neutron moderating material. Alternatively, devices have been constructed in a generally planar geometry with the detecting element disposed behind a plate of moderating material. In some instances, several layers of planar neutron detecting elements have been sandwiched in moderator; although this represents an improvement over the previous cases, it is not ideal.
U.S. Pat. No. 4,795,910 to Henderson et. al. teaches a radiation-detection/scintillation composite comprising a scintillation matrix that is responsive to the absorption of atomic particles for the release of light energy. A solid phase that is separate therefrom consists of a multiplicity of particulate carriers which contain target nuclei suspended within the matrix which have an index of refraction to light energy which closely matches that of the matrix. The carriers comprise hollow spherical glass shells internally containing heavy helium (3He) target nuclei in a gas phase. The carriers are responsive when subjected to radiation which is then absorbed resulting in the release of the energetic particles which can be measured and plotted.
U.S. Pat. No. 5,659,177 to Schulte et. al. discloses and claims a thermal neutron detector with directional capability based on gadolinium (Gd, a rare-earth element) foils for thermal neutron capture (leading to electron emission) that is placed next to a number of segmented silicon semi-conductor detectors. Multiple layers are used so that the layer closest to a neutron source will produce a higher neutron count rate than one further away from it, due to the further layer being shielded by the closer layer. Schulte describes how the use of multiple sets of panels pointed in different directions can provide full directional coverage.
U.S. Pat. No. 5,680,423 to Perkins et. al. teaches a scintillator for detecting neutrons comprised of optical fibers consisting of SiO2, a thermal neutron capturing substance and a scintillating material in a reduced atmosphere. The fibers are contained in an anoxic atmosphere and are coated with a polymer. Photons generated by interaction with thermal neutrons are trapped within the coated fibers and are directed to photoelectric converters. A measurable electronic signal is generated for each thermal neutron interaction within the fiber. These electronic signals are then manipulated, stored, and interpreted by normal methods to infer the quality and quantity of incident radiation.
U.S. Pat. No. 6,895,089 to Wang teaches a signal splitting methology applied specifically to digital subscriber lines (xDSL). This filter comprises a specific combination of low-pass and high-pass elements, shunt elements, and specific resistor, capacitor, and inductor values that provide appropriate impedance matched inputs and outputs for both DSL and basic wireline telecommunication connection (POTS) equipment to be served on the same incoming subscriber line.
U.S. Pat. No. 6,989,541 to Penn teaches a neutron detector consisting of a neutron counter and a plurality of optical fibers peripherally arrayed around the counter. The optical fibers have a layer of scintillator material deposited on them whereby an incidental fast neutron can transfer kinetic energy to nuclei in one or more of the optical fibers to produce recoil protons. The recoil protons interact with the coating to produce scintillation light that is channeled along the optical fiber or fibers with which the neutron interacted. The slowed neutron passes into the neutron counter where the neutron effects generation of a signal coincident with the light produced in the optical fibers in which the neutron deposited energy
Finally, U.S. Pat. No. 7,919,758 to Stephan et. al. discloses and claims a neutron detector device comprising a neutron moderating material that is divided into four sections within a container, each of the four groups of neutron detecting elements disposed at a substantially different distance from the containers' outside surface so that each of the groups is separated from the outside surface of the container by a substantially different density of said moderating material. At least two of the four groups of detecting elements may be independently addressable, and each said independently addressable group has a substantially different detection response for neutrons entering said moderating material from said outside surface.
In each of the prior art descriptions and claimed inventions regarding neutron detectors, a special material or special detector configuration is required at the initial sensor intake to achieve neutron detection capabilities. In Wang '089, a signal splitter for a specific application is disclosed, but the claimed design focuses on separating the two components of the input signal and is not applicable to passing the same signal to multiple systems with minimum distortion. Nowhere in the prior art is there any teaching or suggestion of a separate neutron measurement device that can be connected to an existing gamma-ray detector so as to receive and process the signal originally destined for the gamma-ray signal path and, without significant distortion, duplicate this signal for neutron-activation analysis.